Phylloscopus sibilatrix. Long wings, short tail, strongly yellow breast, white belly, pale fringes on tertials.
Yep sibilatrix, but some trochilus can have more yellow than others, see http://www.portlandbirdobs.org.uk/aa_latestnews.htm and scroll down
The bright yellow green upperparts are often a good feature
Wing and tail length, with primary tips extending onto the rump, makes this a Wood Warbler, but I would add a note of caution to Michael and Brian about using plumage colour as an I.D. feature on such a strongly over-saturated image.
Doesn't look particularly over-saturated to me; the tree is a Hornbeam Carpinus betulus, and that's the colour its leaves are at this time of year
That image is severely oversaturated, plus the colour tone and hue are way off mark. I can only assume that you either have an un-calibrated monitor or that you need to visit your optician (), or both.
I have European (Common) Hornbeam around me, and now in "full leaf", and I can assure you that they are nowhere near the colour of the specimen in the OP's photo.
I have done what I can to redress the dreadful rendition in the OP - but PS has its limitations, especially with a low resolution JPEG.
Despite losing some detail using noise reduction, at least the colour is now closer to "true" and the bird does actually look like a Wood Warbler. This is not a common species in Algarve but one thing which does make it stand out when seen (apart from structural features already notes above) is its very "clean" (near white plus lemon-yellow) appearance - that impression was not apparent in the original image.
Hi all, Just one point that i have not seen mentioned, when seen from below wood warblers have very broad upper chest which is very obvious even on a lone bird. Seen well wood w in spring is very straight forward.